The present invention relates to a fluid engine, and in particular to a fluid engine suitable for using compressed air to power a toy vehicle such as a toy wheeled vehicle or a toy airplane. The fluid engine is provided with a housing having three linearly disposed cavities in which a linearly movable valve rod is positioned. The fluid input cavity is in continuous fluid communication with a source of compressed air and is separated from the fluid delivery cavity by a wall having a valve opening therein. Similarly, the fluid delivery cavity is in continuous fluid communication with a piston cavity having a movable piston therein and is separated from an open-ended exhaust cavity by a wall having an exhaust opening therein. A piston member is operationally connected to the valve rod to open the valve opening and close the exhaust opening when the piston moves in one direction, thereby allowing fluid communication between the source of compressed air and the piston cavity, and to close the valve opening and open the exhaust opening when the piston member moves in the other direction, thereby allowing fluid communication between the piston cavity and the outer air. Two embodiments of the fluid engine, one using a rotating cam member to control the valve rod and the other employing a network of connected links to perform this function, are disclosed.